Gypsum plasters (plaster of Paris) have been known for a long time and because of their easy workability are desired construction materials for piecework and dressing operations in the plaster field. Besides they have been used for artistic and medicinal patterns as well as, following the recent trends, for prefabricated parts. This versatile inclusion in many areas of use is only possible if the neutral setting properties of the gypsum plaster are so changed that the necessary working time is attained. Thus, for example, workers of plaster gypsum for mechanical casting require that the so-called commencement of stiffening of the gypsum plaster first begin 30 to 60 minutes after the mechanical mixing and applying of the gypsum plaster and that subsequently there remains about an equal time in which the gypsum plaster is still shapable and can be smoothed on the wall. On the contrary, in other processes it is desired that the commencement of stiffening of the gypsum plaster be very long drawn out but that the termination of setting occurs a short time thereafter, see Ullmann, Vol. 8, 3rd edition, pages 97 et seq.
To a limited extent this setting characteristic of the gypsum plaster can be regulated by a modification of the process of production in reference to crystal structure and particle size. In most cases, however, one is dependent upon the addition of so-called setting regulators. They acted in different ways, for the most part very complexly in the setting mechanism. Thus, for example, alcohols and lower fatty acids act as retarders in which they lower the solubility of the anhydrite phases of the gypsum plaster. Other materials, as for example phosphates, produce a film forming precipitate on the CaSO.sub.4 particles which greatly lower the speed of solution of the plaster in the mixing water.
The effectiveness of a large number of commercial setting retarders is in that they act as nucleation inhibitors for the calcium sulfate dihydrate. Also the complexing of the calcium ion plays a certain part in the setting process. Therefore, likewise, a setting retardation of the gypsum plaster can be produced with typical chelate-complex formus such as, for example, diethylenetriamine pentaacetic acid or dicarboxylic acids or polyhydroxy carboxylics. In spite of the numerous systems investigated, however, it is still practically impossible today to be able to conclude from the structure of a chemical compound its effectiveness and ability to be included as a setting regulator, see E. Graf and F. Rausch, Zement-, Kalk-Gips, Vol. 4, page 117 et seq. (1951). It is known that l-tartaric acid causes a clear setting retardation in gypsum plaster even when used in small amounts from 0.01 weight % (based on the weight of the plaster). L-tartaric acid must be regarded as one of the best retarders since good gradation in the retardation value can be obtained with it depending on the amount employed. Besides it shows no undesired effects on strength, adhesive power and expansion properties. According to Knorre German application No. P 2542535.8 filed Sept. 24, 1975 and the corresponding Knorre et al U.S. application Ser. No. 723,173 filed Sept. 14, 1976 entitled "Gypsum Plaster" now U.S. Pat. No. 4,094,693 d, L-tartaric acid (racemic tartaric acid) also shows these good properties as a gypsum aid. The entire disclosure of the Knorre U.S. application is hereby incorporated by reference and relied upon.
To be sure there are situations in which L- or d, l-tartaric acid until now could not be added as setting retarders because they lead to overlong setting times with several gypsums or because with other gypsum plasters the typically long retardation of the final phase of the setting process for tartaric acid is undesired.
In further types of gypsum, namely mechanical plaster gypsum, a still further increase in the effectiveness of tartaric acid as a setting retarder is desired.